Passengers using their "eyes and ears" is the only way the authorities will have "success" in stopping terror threats on the rail network, police have warned.

A nationwide campaign has been launched to encourage travellers to be vigilant on trains and at stations. They are being urged to immediately report anything unusual.

British Transport Police (BTP) Assistant Chief Constable Alun Thomas told the Press Association: "The rail network is interconnected right across the UK.

"The six million-plus rail journeys a day provide a crowded environment, an accessible environment and one that we are very proud of as an icon of British industry.

"Therefore it can be seen as attractive as a target.

"Hearts and minds, eyes and ears are crucial to us. The expansion of our intelligence and information system through the public is the only thing that will provide us with success as we move into a threat environment that is accelerating to one that would have been incomprehensible just a few years ago."

On October 20 a security alert was sparked after an explosive device was found on a Tube train in London, prompting the evacuation of North Greenwich Underground station.

Rail minister Paul Maynard said: "We want to send a clear message to anyone threatening the security of the rail network that there are thousands of pairs of eyes and ears ready to report any potential threat to the British Transport Police and rail staff who are ready to respond to these reports.

"Today's campaign is aimed at our railways but the recent incident at North Greenwich Underground station, where an alert member of the public reported an unattended bag to a member of staff, reminds us just how important it is to be vigilant."

Everyone who uses the rail network is being urged to report anything unusual either in person to rail staff or by contacting BTP by text message on 61016 or calling 0800 405040.

The "See It. Say It. Sorted" campaign was launched at major railway stations across Britain, including London Waterloo, Manchester Piccadilly and Glasgow Central.

Andrew Parker, the head of MI5, said on Monday that the terror threat from Islamic State is "here to stay" and described the danger as "at least a generational challenge".

The official threat level for international terrorism in the UK currently stands at severe, meaning an attack is ''highly likely''.

Mark Rowley, national policing lead for counter-terrorism, said that half the foiled terror plots in the UK in the past two years involved a plan to buy guns.

Authorities fear that links between street gangs and terrorists could give extremists access to weapons and risk the type of mass shootings seen in the Paris attacks.